Optical system having a diffractive optical element, and optical apparatus

ABSTRACT

An optical system includes a diffractive optical element having a diffraction grating provided, on a lens surface having a curvature, in a concentric-circles shape rotationally-symmetrical with respect to an optical axis. The sign of the curvature of the lens surface having the diffraction grating provided thereon is the same as the sign of a focal length, at a design wavelength, of a system composed of, in the optical system, a surface disposed nearest to an object side to a surface disposed immediately before the lens surface having the diffraction grating provided thereon, and is different from the sign of the distance from the optical axis to a position where the center ray of an off-axial light flux enters the lens surface having the diffraction grating provided thereon. Further, the apex of an imaginary cone formed by extending a non-effective surface of the diffraction grating is located adjacent to the center of curvature of the lens surface having the diffraction grating provided thereon.

This application is a division of application Ser. No. 09/799,058 filedMar. 6, 2001, now U.S. Pat. No. 6,473,232.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical system having a diffractiveoptical element, and more particularly to an optical system suited tooptical apparatuses, such as film cameras, video cameras, digitalcameras, telescopes, projectors, etc., in which a diffractive opticalelement and a refracting optical element are combined to effectachromatism well.

2. Description of Related Art

Heretofore, as one of methods for correcting chromatic aberration of anoptical system, there is a method of combining two glass materials(lenses) which differ in dispersion from each other.

As against such a conventional method of combining the two glassmaterials to diminish chromatic aberration, there is a method fordiminishing chromatic aberration by providing, at a lens surface or apart of an optical system, a diffractive optical element, such as adiffraction grating, having a diffracting function, as disclosed in SPIEVol. 1354 International Lens Design Conference (1990), JapaneseLaid-Open Patent Application No. Hei 4-213421 (corresponding to U.S.Pat. No. 5,044,706), Japanese Laid-Open Patent Application No. Hei6-324262 (corresponding to U.S. Pat. No. 5,790,321), U.S. Pat. No.5,044,706, etc.

This method is based on the utilization of the physical phenomenon thata refractive surface and a diffractive surface in an optical systemcause the behavior of chromatic aberration with respect to a ray oflight of a certain reference wavelength to occur in respective oppositedirections. Further, it is possible to make such a diffractive opticalelement have an aspheric-lens-like effect by varying the period of theperiodic structure thereof, thereby greatly effectively loweringaberration.

Here, while, in the case of refraction, one ray of light remains beingone ray of light even after being refracted, one ray of light, in thecase of diffraction, is divided into a number of rays of various ordersafter being diffracted. Therefore, in a case where a diffractive opticalelement is used in an optical system, it is necessary that the gratingstructure of the diffractive optical element is decided in such a mannerthat light fluxes included in a useful wavelength region concentrate onone particular order (hereinafter referred to also as a design order),and it is necessary that the diffractive optical element has adiffraction efficiency excellent over the entire image plane.

With regard to the diffraction grating, there is proposed, in JapaneseLaid-Open Patent Application No. Hei 10-268115 (corresponding to U.S.Pat. No. 5,995,286), an optical system using a diffraction grating ofthe blazed shape to aim at the evenness of the diffraction efficiencyover the entire observation image plane.

In the above Japanese-Laid-Open Patent Application No. Hei 10-268115,there are disclosed a Keplerian viewfinder optical system arranged suchthat a diffraction grating of the blazed shape in which the height of agrating part at a marginal area of the diffraction grating is less thanthe depth of a grating part at a central area, around an optical axis,of the diffraction grating is used to make the diffraction efficiency atthe central area approximately equal to that at the marginal area, and aKeplerian viewfinder optical system arranged such that a diffractiongrating of the blazed shape in which a non-effective surface (a surfacehaving no diffracting function in the diffraction At grating andcorresponding to a side surface of the diffraction grating) in a centralarea around an optical axis is formed as a part of a cylindrical surfaceand a non-effective surface of a marginal area is formed as a part of aconical surface is used to prevent the shading of a ray of light at thenon-effective surface in the marginal area.

In the Keplerian viewfinder optical systems as proposed in the aboveJapanese Laid-Open Patent Application No. Hei 10-268115, an on-axiallight flux and a most off-axial light flux are separate from each otherwhen passing through the position of a diffractive optical surfaceprovided at a position relatively distant from a stop. Accordingly, thisconstruction has such a characteristic that, without the above state ofpassing-through of rays of light, it is impossible to obtain the effectof evenness of the diffraction efficiency including the shading of a rayof light.

On the other hand, in a photographing optical system to which an opticalsystem of the invention is assumed to be applicable, as isunderstandable from FIGS. 1, 2 and 3, which are used for the descriptionof embodiments of the invention, an on-axial light flux and an off-axiallight flux are relatively unseparate from each other in the interior ofthe optical system. Accordingly, even on a lens surface distant from astop, the area of passing-through of the on-axial light flux and that ofthe off-axial light flux have a tendency to overlap each other.

Therefore, even if the structural arrangement disclosed in the aboveJapanese Laid-Open Patent Application No. Hei 10-268115 is applied to aphotographic lens as it stands, it is difficult to obtain a diffractionefficiency excellent concurrently with respect to both the on-axiallight flux and the off-axial light flux.

In particular, in the case of a photographic lens having arelatively-large aperture, since the respective areas, on which anon-axial light flux and an off-axial light flux are made incident, of alens surface having a diffraction grating provided thereon overlap eachother greatly, the above-mentioned difficulty increases.

BRIEF SUMMARY OF THE INVENTION

It is an object of the invention to provide an optical system havingsuch high optical performance that, when effecting achromatism bycombining a diffractive optical element and a refractive opticalelement, a diffraction efficiency excellent over the entire image planecan be obtained even if light fluxes which are to reach respectivepositions of the image plane overlap each other greatly on a diffractiveoptical surface.

To attain the above object, in accordance with an aspect of theinvention, there is provided an optical system, comprising a diffractiveoptical element having a diffraction grating provided, on a lens surfacehaving a curvature, in a concentric-circles shape rotationallysymmetrical with respect to an optical axis, wherein the sign of thecurvature of the lens surface having the diffraction grating providedthereon is the same as the sign of a focal length, at a designwavelength, of a system composed of, in the optical system, a surfacedisposed nearest to an object side to a surface disposed immediatelybefore the lens surface having the diffraction grating provided thereon,and is different from the sign of a distance from the optical axis to aposition where a center ray of an off-axial light flux enters the lenssurface having the diffraction grating provided thereon.

Here, the sign of the curvature of the lens surface is consideredpositive if the center of curvature exists on a light-exit side (imageside) with respect to the lens surface, and is considered negative ifthe center of curvature exists on a light-entrance side (object side)with respect to the lens surface. Accordingly, the curvature of a lenssurface convex facing the object side (concave facing the image side)has a positive sign, and the curvature of a lens surface concave facingthe object side (convex facing the image side) has a negative sign. Onthe other hand, the sign of the distance from the optical axis to theposition where a center ray of an off-axial light flux enters the lenssurface having the diffraction grating provided thereon is consideredpositive if the position where the center ray enters the lens surfaceexists on a side opposite to a side from which the center ray enters theoptical system with respect to the optical axis, and is considerednegative if the position where the center ray enters the lens surfaceexists on the same side as a side from which the center ray enters theoptical system with respect to the optical axis. Accordingly, thedistance from the optical axis to the position where a center ray of anoff-axial light flux enters the lens surface has a negative sign if thecenter ray enters the lens surface before crossing the optical axis, andhas a positive sign if the center ray enters the lens surface aftercrossing the optical axis.

Further, in accordance with another aspect of the invention, there isprovided an optical system, comprising a diffractive optical elementhaving a diffraction grating provided, on a lens surface havingcurvature, in a concentric-circles shape rotationally-symmetrical withrespect to an optical axis, wherein an apex of an imaginary cone formedby extending a non-effective surface of the diffraction grating (a sidesurface of the diffraction grating) is located adjacent to the center ofcurvature of the lens surface having the diffraction grating providedthereon.

These and further objects and features of the invention will becomeapparent from the following detailed description of preferredembodiments thereof taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a sectional view of an optical system according to a firstembodiment of the invention.

FIG. 2 is a sectional view of an optical system according to a secondembodiment of the invention.

FIG. 3 is a sectional view of an optical system according to a thirdembodiment of the invention.

FIG. 4 is a sectional showing, in outline, a diffractive optical surfacein which both a diffraction grating and a lens surface having thediffraction grating provided thereon are positive in optical power.

FIG. 5 is a schematic diagram showing, in outline, a diffractive opticalsurface in which both a diffraction grating and a lens surface havingthe diffraction grating provided thereon are negative in optical power.

FIG. 6 is a sectional view showing, in outline, a single-layerdiffraction grating.

FIG. 7 is a graph showing the wavelength-dependent characteristic of thediffraction efficiency of the diffraction grating shown in FIG. 6.

FIG. 8 is a sectional view showing, in outline, a laminated diffractiongrating (of the close-contact laminated type).

FIG. 9 is a graph showing the wavelength-dependent characteristic of thediffraction efficiency of the diffraction grating shown in FIG. 8.

FIG. 10 is a sectional view showing, in outline, another laminateddiffraction grating (of the adjacently-laminated type with an airlayer).

FIG. 11 is a table showing angles of incidence (θ), on a diffractiongrating, of a center ray and marginal rays (upper and lower rays) amongmeridional rays of an on-axial light flux and an off-axial light fluxentering the optical system according to the first embodiment of theinvention.

FIG. 12 shows graphs for the grating-pitch-dependent characteristics ofthe diffraction efficiency, at wavelengths 450 nm, 550 nm and 650 nm,for a first-order diffracted light including shading at a non-effectivesurface with respect to incident rays of light having the angles ofincidence θ=0°, θ=−10° and θ=+10° in the diffraction grating of theadjacently-laminated type of the optical system according to the firstembodiment.

FIG. 13 is a schematic diagram showing, in outline, the arrangement ofan optical apparatus (a single-lens-reflex camera) according to afurther embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, preferred embodiments of the invention will be described indetail with reference to the drawings.

FIGS. 1, 2 and 3 are sectional views showing the essential parts ofoptical systems OL according to first, second and third embodiments ofthe invention, respectively. In the examples shown in FIGS. 1, 2 and 3,the invention is applied to the photographing optical systems OL of thetelephoto type, of the Gauss type and of the inverted telephoto type,respectively. In FIGS. 1, 2 and 3, reference character DO denotes adiffractive optical surface which is a lens surface having a diffractiongrating provided thereon. The diffraction grating is formed in aconcentric-circles shape rotationally-symmetrical with respect to anoptical axis, and the form of a section of the diffraction grating isthe blazed type. Reference character SP denotes an aperture stop fordetermining the brightness of the optical system OL, and referencecharacter IP denotes an image plane.

FIGS. 4 and 5 are diagrams for explaining the diffractive opticalsurface according to the invention. In the state shown in each of FIGS.4 and 5, a diffraction grating 2 of the blazed type is provided on alens surface la on one side of a lens 1. In FIG. 4, reference numeral 4denotes an effective surface having a desired diffracting function ofthe diffraction grating, and reference numeral 3 denotes a side surfacewhich is obtained when forming the diffraction grating 2 of the blazedtype on the lens surface 1 a and does not take part in the diffractingfunction (hereinafter referred to as the “non-effective surface”).

The non-effective surface 3, when being imaginarily extended, intersectsan optical axis 5 at one point, and is a part of the conical surface ofan imaginary cone thus formed with such an intersection point taken asan apex thereof. In FIG. 4, the apex of such an imaginary cone isdenoted by reference character DOP, and the center of curvature of thelens surface 1 a is denoted by reference character RC.

In each of the first to third embodiments, the apex DOP is made to belocated adjacent to the center of curvature RC. Here, the locationadjacent to the center of curvature RC means that the followingcondition is satisfied:

|DL/R|<0.3

where DL is the distance from the apex DOP of the imaginary cone to thecenter of curvature of the lens surface 1 a, which is a base having thediffraction grating provided thereon, and R is the radius of curvatureof the lens surface 1 a.

In each of the first to third embodiments, the directions of incidenceof rays from an on-axial object point (an on-axial light flux) on thelens surface, which is a base having the diffraction grating providedthereon, are respectively made approximately equal to the directions ofnormal lines at intersection points where the rays intersect the lenssurface having the diffraction grating provided thereon. For example,the height of each zone of the diffraction grating is set to such aheight that the diffraction efficiency of a ray of light coincident withthe optical axis becomes 100% (such a height that an optical path lengthbecomes an integer times the design wavelength), and is made uniform inthe directions of the normal lines of the lens surface having thediffraction grating provided thereon. This arrangement makes it possibleto set the diffraction efficiency for the on-axial light flux to almost100%.

As a result, shading of the whole on-axial light flux is prevented, sothat it is possible to heighten the diffraction efficiency.

First, the diffraction efficiency for the on-axial light flux amonglight fluxes to be diffracted by the diffractive optical surface will bedescribed.

In each of the first to third embodiments, the sign of the curvature Ra(i.e, 1/R) of the lens surface having the diffraction grating providedthereon (for example, the sign of the curvature of a concave surfacefacing the image side (the light-exit side) being positive and the signof the curvature of a convex surface facing the image side beingnegative) is made to be the same as the sign of a focal length “fa”, atthe design wavelength, of a composite system composed of a surfacedisposed nearest to the object side (the light-entrance side) to asurface disposed immediately before the lens surface having thediffraction grating provided thereon in the optical system. In the caseof the first embodiment shown in FIG. 1, both the curvature Ra and thefocal length “fa” have a positive sign, and in the cases of the secondand third embodiments shown in FIGS. 2 and 3, both the curvature Ra andthe focal length “fa” have a negative sign.

In this instance, the directions of incidence in which rays of lightconstituting the on-axial light flux reaching the center of the imageplane enter the lens surface having the diffraction grating providedthereon respectively become approximately equal to the directions ofnormal lines at intersection points where the rays of light intersectthe lens surface having the diffraction grating provided thereon. Insuch a state, the apex DOP of the imaginary cone, a part of which isformed by the non-effective surface of the diffraction grating, is madeto be located adjacent to the center of curvature RC (the position onwhich the normal lines concentrate) of the lens surface having thediffraction grating provided thereon. By this arrangement, the shadingof on-axial rays of light at the non-effective surface of thediffraction grating is lessened, so that the diffraction efficiency isprevented from lowering due to shading.

Next, the diffraction efficiency for the off-axial light flux amonglight fluxes to be diffracted by the diffractive optical surface will bedescribed.

In each of the first to third embodiments, the sign of the distance Hafrom the optical axis to a position where the center ray of theoff-axial light flux enters the lens surface having the diffractiongrating provided thereon (the sign of the distance Ha is consideredpositive if the position of incidence on the lens surface is on a sideopposite to the side from which the center ray enters the optical systemwith respect to the optical axis, and is considered negative if theposition of incidence on the lens surface is on the same side as theside from which the center ray enters the optical system with respect tothe optical axis, and, accordingly, the distance Ha has a positive signif the center ray enters the lens surface after crossing the opticalaxis.) is made to be different from the sign of the curvature Ra of thelens surface having the diffraction grating provided thereon. In thecase of the first embodiment, Ha<O and Ra>O and in the cases of thesecond and third embodiments, Ha>0 and Ra<0.

In this instance, similarly to the case of the on-axial light flux, thedirections of incidence in which rays of light constituting theoff-axial light flux reaching the margin of the image plane enter thelens surface having the diffraction grating provided thereonrespectively become approximately equal to the directions of normallines at intersection points where the rays of light intersect the lenssurface having the diffraction grating provided thereon.

Accordingly, similarly to the case of the on-axial light flux, the apexDOP of the imaginary cone, a part of which is formed by thenon-effective surface 3 of the diffraction grating, is made to belocated adjacent to the center of curvature RC (the position on whichthe normal lines concentrate) of the lens surface having the diffractiongrating provided thereon. By this arrangement, the shading of rays oflight of the off-axial light flux reaching the margin of the image planeat the non-effective surface of the diffraction grating is lessened, sothat the diffraction efficiency can be prevented from lowering.

Further, the directions of incidence in which rays of light constitutingthe off-axial light flux reaching the margin of the image plane enterthe lens surface having the diffraction grating provided thereonrespectively become approximately equal to the directions of normallines at intersection points where the rays of light intersect the lenssurface having the diffraction grating provided thereon. Therefore, itis possible to obtain a high diffraction efficiency even while keepingthe height of the zone of the diffraction grating decided on the basisof the on-axial light flux as mentioned in the foregoing.

According to the invention, with the arrangement as described above, itis possible to lessen the shading over the entire image plane, thusobtaining a high diffraction efficiency, and, in particular, to mitigatethe adverse effect of color flare occurring due to unnecessarydiffracted rays of color light fluxes at the time of photographing ahigh-luminance object.

While, in the invention, with the various elements thereof defined asdescribed above, there is attained an optical system having good opticalperformance over the entire image plane, it is preferable to furthersatisfy at least one of the following structural conditions.

With such a condition satisfied, it is possible to further lessenshading, at the non-effective surface of the diffraction grating, of alight flux reaching the center or thereabout of the image plane, whichis of relatively great importance in image quality, and it is possibleto further improve the diffraction efficiency for a light flux reachingthe center or thereabout of the image plane.

(A-1) The following condition is satisfied:

 |D/R|<5  (1)

where D is a distance from the position of the center of curvature ofthe lens surface having the diffraction grating provided thereon to theposition of a focus, at the design wavelength, of a combined systemcomposed of a surface disposed nearest to the object side to a surfacedisposed immediately before the lens surface having the diffractiongrating provided thereon in the optical system, and R is the radius ofcurvature of the lens surface having the diffraction grating providedthereon.

If the upper limit of the condition (1) is exceeded, the diffractionefficiency, in particular, around the center of the image planedeteriorates disadvantageously. In the invention, it is more preferableto alter the numerical range of the condition (1) as follows:

|D/R|<3  (1)′

thereby further improving the diffraction efficiency.

(A-2) The following condition is satisfied:

C ₁·P <0  (2)

where P is a refractive power of the lens surface having the diffractiongrating provided thereon, and C₁ is a phase coefficient for asecond-degree term when a phase shape of the diffraction grating isexpressed by the following equation:

φ(Y)=(2π/λ_(O))(C ₁ Y ² +C ₂ Y ⁴ +C ₃ Y ⁶+. . . )  (a)

where Y is the height in the vertical direction from the optical axis,λ_(O) is the design wavelength, and C_(i) is a phase coefficient (i=1,2, 3 . . . ).

The condition (2) is provided for manufacturing the diffraction gratingaccording to the invention with high accuracy.

The technical significance of the condition (2) will be described below.

An optical power φ_(D)(λ, m) of the diffractive surface (correspondingto a refractive power and expressed by the reciprocal of the focallength) for an arbitrary wavelength λ and an arbitrary diffraction ordercan be expressed by the following equation, using the phase coefficientC, in the above equation (a):

φ_(D)(λ, m)=−2C ₁ mλ/m _(O)λ_(O)  (b).

Thus, an optical power of the diffractive surface for the designwavelength λ_(O) and the design diffraction order m_(O) becomes asfollows:

φ_(D)(λ_(O) , m _(O))=−2C ₁  (c).

In other words, the condition (2) means that the optical powerφ_(D)(λ_(O), m_(O)) of the diffraction grating for the design wavelengthand the design diffraction order is set to have the same sign as thesign of the refractive power of the lens surface having the diffractiongrating provided thereon.

Further, the significance which such an arrangement gives in theinvention will be described below on the basis of two kinds of blazedshapes, between which the sign of the phase coefficient C₁ varies, withreference to FIGS. 4 and 5.

FIGS. 4 and 5 are sectional views respectively showing, in outline,diffraction gratings which are formed into the different blazed shapes,between which the sign of the phase coefficient C₁ varies, on the basisof the condition (1). In each of the diffraction gratings shown in FIGS.4 and 5, the sign of the refractive power P of the lens surface 1 ahaving the diffraction grating 2 provided thereon is set in such a wayas to satisfy the condition (2). In the case of the diffraction gratingshown in FIG. 4, C₁<0 (the optical power of the diffraction gratinghaving a positive value), and P>0. In the case of the diffractiongrating shown in FIG. 5, C₁>0 (the optical power of the diffractiongrating having a negative value), and P<0. As mentioned in theforegoing, in FIG. 4, the diffraction grating 2 is provided on the lenssurface 1 a of the lens 1, and the apex DOP of the imaginary cone formedby the non-effective surface 3 of the diffraction grating 2 is locatedadjacent to the center of curvature RC of the lens surface 1 a havingthe diffraction grating 2 provided thereon.

As the methods for manufacturing diffraction gratings of such a blazedshape, there are a method of performing press molding with a mold or thelike while fusing glass at a high temperature, a method of performingpress molding of ultraviolet-curable plastic or the like with a mold onthe surface of a glass substrate or the like and curing the plastic withradiation of an ultraviolet ray, a method of molding plastic with a moldtogether with a lens, etc. Further, there are a method of forming adiffraction grating by directly cutting glass, and a method of forming adiffraction grating of the finely-stepped shape by wet-etching ordry-etching S_(i)O₂ or the like.

As is apparent also from FIGS. 4 and 5, since the non-effective surface3 of the diffraction grating 2 is a part of the conical surface, if thesign of the refractive power P of the lens surface having thediffraction grating provided thereon or the sign of the optical power ofthe diffraction grating is made opposite to that adopted in thediffraction grating shown in FIG. 4 or 5, i.e., if C₁·P>0, there arisevarious problems in terms of workability. For example, in using a mold,it is difficult to work the mold, and, at the time of transfer on themold, it is difficult for fused glass or plastic to intrude in thedirection of the depth of the grating, thus causing the transferabilityto deteriorate. Further, at the time of the mold release, in the methodof performing molding while fusing glass, the method of molding plasticwith a mold together with a lens, etc., the mold can not be released inthe worst state, and, at the most, a tip portion of the grating isdamaged, thus causing the diffraction efficiency to deteriorate. In themethod of transferring ultraviolet-curable plastic to a glass substrateor the like, while, since the viscosity of the plastic is relativelylow, the mold may be somehow released, a tip portion or thereabout ofthe grating is deformed due to the stress occurring at the time of themold release, thus also causing the diffraction efficiency todeteriorate.

In short, in the case of C₁·P>0, in whatever forming-method among theforming methods using a mold, the transferability of the mold, thereleasability of the mold, etc., are deteriorated and it becomesimpossible to obtain the desired diffraction efficiency. Therefore, itis preferable to satisfy the above condition (2).

(A-3) The diffraction grating is a diffraction grating of the laminatedtype (a laminated diffraction grating).

(A-4) The laminated diffraction grating is a diffraction grating of theadjacently-laminated type (an adjacently-laminated diffraction grating)in which at least one thin air layer is included and two diffractiongratings are disposed adjacent to each other across the thin air layer.

(A-5) The adjacently-laminated diffraction grating is provided betweentwo adjacent lens surfaces having substantially the same curvature, andis composed of three layers, i.e., in order from the object side, afirst layer, a second layer and a third layer, and the second layer isthe thin air layer.

(A-6) The first layer and the third layer of the adjacently-laminateddiffraction grating are formed with ultraviolet-curable plastic.

The above-mentioned structural conditions (A-3) to (A-6) are providedfor defining the grating structure for heightening the diffractionefficiency over the entirety of the useful wavelength region. Each ofthe structural conditions (A-3) to (A-6) will be described in detailbelow.

As the method of heightening the absolute value of the diffractionefficiency over the entirety of the useful wavelength region, there isknown a diffraction grating of the laminated type in which a pluralityof blazed-type diffraction gratings are disposed in close contact witheach other or adjacent to each other and the refractive indices and Abbenumbers of the materials of the respective diffraction gratings, thedepths of the gratings, etc., are appropriately set.

FIG. 7 is a graph showing the wavelength-dependent characteristic of thediffraction efficiency, mainly for the first-order diffracted light,obtained when a light flux is made to vertically enter the single-layerblazed-type diffraction grating shown in FIG. 6. In the actual structureof the diffraction-grating, as shown in FIG. 6, ultraviolet-curableplastic is coated on the surface of a base material 11 to form a plasticportions and, on the plastic portion, there is formed a grating 12having a grating thickness d arranged such that the diffractionefficiency for the first-order diffracted light at the wavelength of 530nm becomes 100%. As is apparent from FIG. 7, the diffraction efficiencyfor the design order decreases accordingly as the wavelength goes awayfrom the optimized wavelength of 530 nm, and, conversely, the diffractedlight of the zero order and the second order near the design orderincreases. Such an increase of the diffracted light of the order otherthan the design order causes flare, thereby lowering the resolution ofthe optical system.

On the other hand, FIG. 9 is a graph showing the wavelength-dependentcharacteristic of the diffraction efficiency, mainly for the first-orderdiffracted light, obtained when a light flux is made to vertically enterthe diffraction grating of the close-contact laminated type shown inFIG. 8. In the specific structure of the diffraction grating, as shownin FIG. 8, a first diffraction grating 13 made from ultraviolet-curableplastic (nd=1.499, νd=54) is formed on a base material 11, and, on thefirst diffraction grating 13, there is formed a second diffractiongrating 14 made from another ultraviolet-curable plastic (nd=1.598,νd=28). In such a combination of those materials, the grating thicknessd1 of the first diffraction grating 13 is set to 13.8 μm, and thegrating thickness d2 of the second diffraction grating 14 is set to 10.5μm. As is apparent from FIG. 9, with the diffraction grating having thislaminated structure, the diffraction efficiency for the design orderbecomes 95% or more over the entirety of the useful wavelength region.

As mentioned in the foregoing, with the diffraction grating of thelaminated structure used as a diffraction grating according to each ofthe embodiments, it is possible to further improve optical performance.At the same time, with the above-described construction, it is possibleto obtain a diffraction efficiency excellent in wavelength-dependentcharacteristic over the entire image plane. Therefore, it is preferableto use the diffraction grating of the laminated structure.

Next, the influence of the non-effective surface of the diffractiongrating on the diffraction efficiency will be described below withrespect to, among diffraction gratings of a laminated structure, adiffraction grating of the close-contact laminated type in which gratingportions are in close contact with each other, and a diffraction gratingof the adjacently-laminated type in which grating portions are disposedadjacent to each other across a thin air layer.

In a case where the above two types of diffraction gratings are made ofthe same material, since the depth of a grating portion required forobtaining the necessary diffracted light is inversely proportional tothe absolute value of the difference of refractive indices of surfaceson the entrance side and the exit side of the grating portion, thediffraction grating of the adjacently-laminated type, which has an airlayer, makes it possible to reduce the total of depths of the wholegrating portion more than the diffraction grating of the close-contactlaminated type.

As a result, the diffraction grating of the adjacently-laminated typelessens the shading of rays on the non-effective surface more than thediffraction grating of the close-contact laminated type, and is,therefore, advantageous with respect of an improvement in thediffraction efficiency.

Therefore, it is preferable to use the diffraction grating of theadjacently-laminated type, in which grating portions are disposedadjacent to each other across a thin air layer.

In this instance, if a lens, which is a component of the optical system,is so divided into two parts that lens surfaces obtained by thisdivision have approximately the same curvature, and the diffractiongrating of the adjacently-laminated type is provided on each of the lenssurfaces, with the result that there are formed three layers, i.e., afirst layer, a second layer and a third layer in order from the objectside, the second layer being a thin air layer, it is possible toincrease the diffraction efficiency with a relatively simpleconstruction and without influencing the various aberrations of theentire optical system.

In a case where one lens is so divided into two parts that lens surfacesobtained by this division have approximately the same curvature, thecurvatures of the lens surfaces hardly contribute to the variousaberrations even if they are set to any values while being keptapproximately the same. Therefore, it is possible to set the curvaturesof the lens surfaces to values which are specialized for optimizing thediffraction efficiency around the center of the image plane and aroundthe margin of the image plane. Accordingly, it is preferable to use thediffraction grating of the adjacently-laminated type having such aconstruction.

In this instance, the glass materials of the divided lens parts need notbe the same, and may be varied according to necessity. In addition, iftwo lens surfaces, on each of which the diffraction grating is to beprovided, exist from the beginning, before the diffraction grating isprovided, for reasons of the structure for correcting aberration, suchas chromatic aberration, spherical aberration or coma, and are suchcemented surfaces that the conditions of curvature, etc., satisfy thearrangement of the invention, it is unnecessary to newly divide a lens,and the diffraction grating ought to be provided on the existing lenssurfaces.

FIG. 10 is an enlarged sectional view showing, in outline, a part of adiffraction grating of the adjacently-laminated type which is applied tothe optical system of the first embodiment. In FIG. 10, referencenumerals 21 and 22 denote lens parts into which one lens is so dividedthat lens surfaces obtained by this division have approximately the samecurvature. Reference numerals 23 and 24 denote a diffraction gratingserving as a first layer (nd=1.6685, νd=19.7) and a diffraction gratingserving as a third layer (nd=1.5240, νd=50.8), respectively. Referencenumeral 25 denotes an air layer serving as a second layer. Referencecharacter O denotes an optical axis of the optical system. Further,reference characters P and Q denote rays incident on the diffractiongrating 23. Reference character V denotes a normal line at anintersection point between the ray P (Q) and a lens surface 21 a havingthe diffraction grating 23 provided thereon. Reference character edenotes an angle which the ray P (Q) makes with the normal line V of thelens surface 21 a, with a positive angle being taken clockwise in FIG.10. Further, reference characters d1 and d2 denote the gratingthicknesses of the first layer and the third layer, respectively, beingset to d1=5 μm and d2=7.5 μm. Reference character Pi denotes the gratingpitch of the i-th zone of the first layer (the grating pitch of the i-thzone of the first layer being set equal to that of the third layer). Theminimum grating pitch, which is on the 155th zone at the most marginalportion of the diffraction grating, is 156 μm.

Incidentally, a diffraction grating proposed in Japanese PatentApplication No. Hei 11-213374 is applicable to the shape of thediffraction grating of the adjacently-laminated type.

FIG. 11 is a table showing angles of incidence (θ), on the diffractiongrating, of a center ray and marginal rays (upper and lower rays) amongmeridional rays of an on-axial light flux and an off-axial light fluxentering the optical system according to the first embodiment. Further,FIG. 12 shows graphs for the grating-pitch-dependent characteristics ofthe diffraction efficiency, in wavelengths 450 nm, 550 nm and 650 nm,for a first-order diffracted light including shading at thenon-effective surface of the diffraction grating with respect toincident rays of light having the angles of incidence θ=0°, θ=−10° andθ=+10° in the diffraction grating of the adjacently-laminated type shownin FIG. 10.

In general, the smaller the grating pitch, the more greatly a convertedoptical path length of a ray passing through the grating portion varieswith respect to a change of the angle of incidence of the ray, and,therefore, the more remarkable the deterioration of the diffractionefficiency becomes. As shown in FIG. 12, the diffraction efficiency ofthe diffraction grating of the adjacently-laminated type that is appliedto the optical system according to the first embodiment is excellent andincludes a wavelength-dependent characteristic.

Then, as shown in FIG. 11, the angle of incidence θ of array in theoptical system of the first embodiment takes the largest absolute valuefor the marginal ray (lower ray) of the maximum angle of view (the halfangle of view being 3.16°), i.e., θ=+2.50°. Further, the position wherethe marginal ray passes through is the most marginal part of the gratingportion, and, as mentioned in the foregoing, the grating pitch of thegrating portion becomes a minimum value Pi=156 μm (i=155) at such aposition. While the diffraction efficiency for a ray passing throughthat position becomes lowest, it is understood from FIG. 12 that thehigh efficiency equivalent to the diffraction efficiency for an incidentray having the angle of incidence θ=0° can be kept.

Next, the method for forming the laminated-type diffraction grating andthe degree of freedom of design for the aberration correction and thediffraction efficiency will be described.

One of purposes of introducing the diffraction grating to an opticalsystem having a refractive member, such as a lens, is to cancel, withthe diffraction grating, chromatic aberration occurring at therefractive optical system.

Accordingly, in a case where the diffraction grating is to be providedon the surface of a lens, since the lens takes partial charge ofchromatic aberration required as a refractive optical system withrespect to the partial charge of chromatic aberration by the diffractiongrating, it is difficult to arbitrarily select the material of the lens.

More specifically, in the case of the method of press-molding, with amold or the like, the diffraction grating together with a lens whilefusing glass at a high temperature, or in the case of the method ofmolding, with a mold, the diffraction grating together with a lens usingplastic, the lens and the diffraction grating become made of the samematerial, so that the degree of freedom of selecting the material of thediffraction grating is lost. Therefore, it becomes difficult to make thecorrection of chromatic aberration compatible with the improvement ofthe wavelength-dependent characteristic of the diffraction efficiency inthe laminated-type diffraction grating.

Accordingly, if such a method as to enable the materials of the lens andthe diffraction grating to be selected independent of each other, forexample, the method of forming the diffraction grating withultraviolet-curable plastic, is used, it becomes possible to obtain anoptical system excellent both in the correction of chromatic aberrationand the diffraction efficiency. Therefore, such a forming method oughtto be used.

In addition, the optical system according to the invention is applicablewidely to an image pickup apparatus, such as a film camera, a videocamera, a digital camera or the like, an observation apparatus, such asa telescope, a binocular or the like, a stepper (a projection exposureapparatus) for manufacturing semiconductor devices, a variety of opticalmeasuring apparatuses, etc.

Here, an embodiment in which the optical system according to each of thefirst to third embodiments is applied to an optical apparatus will bedescribed with reference to FIG. 13.

FIG. 13 is a schematic diagram showing, in outline, the essential partsof a single-lens reflex camera. In FIG. 13, reference numeral 10 denotesa photographic lens having an optical system 1 according to any one ofthe first to third embodiments. The optical system 1 is held by a lensbarrel 2 serving as a holding member. Reference numeral 20 denotes acamera body, which is composed of a quick-return mirror 3 arranged toreflect upward a light flux coming from the photographic lens 10, afocusing screen 4 disposed on the image forming position of thephotographic lens 10, a pentagonal roof prism 5 arranged to convert aninverted image formed on the focusing screen 4 into an erecting image,an eyepiece lens 6 provided for observing the erecting image, etc.Reference numeral 7 denotes a film surface. In taking a picture, thequick-return mirror 3 retreats from the optical path and a shutter (notshown) is opened, so that an image is formed on the film surface 7 bythe photographic lens 10.

The advantageous effects mentioned in each of the first to thirdembodiments are effectively enjoyed by such an optical apparatus asdisclosed in the present embodiment.

Next, numerical data of the numerical examples 1 to 3 of optical systemscorresponding to the first to third embodiments of the invention areshown.

In the numerical data of the numerical examples 1 to 3, f denotes thefocal length, Fno denotes the F-number, ω denotes a half angle of view,ri denotes the radius of curvature of the i-th surface, when countedfrom the object side, di denotes the separation between the i-th surfaceand the (i+1)th surface, when counted from the object side, ni and νirespectively denote the refractive index and Abbe number of the i-thoptical member, when counted from the object side.

The shape of an aspheric surface is expressed in the coordinates with anX axis in the optical axis direction (the direction in which lightadvances) and an H axis in the direction perpendicular to the opticalaxis, with the intersection point between the aspheric surface and the Xaxis taken as the original point, by the following equation:$X = {\frac{H^{2}/r}{1 + \sqrt{1 - ( {H/r} )^{2}}} + {A\quad H^{2}} + {B\quad H^{4}} + {C\quad H^{6}} + {D\quad H^{8}} + {E\quad H^{10}} + {F\quad H^{12}}}$

where r is the radius of curvature of a paraxial portion of the asphericsurface, and A, B, C, D, E and F are aspheric coefficients.

Further, the coefficients C₁, C₂ . . . of the shape of the diffractiongrating are shown on the basis of the above-mentioned equation (a). Inaddition, the indication “D-X” means “×10^(−X)”.

In addition, the values of the factors in the above-mentioned conditionsfor the numerical examples 1 to 3 are listed in Table-1.

Numerical Example 1: f = 392.00  Fno = 1:4.12  2ω = 6.32° r 1 = 115.683d 1 = 9.40 n 1 = 1.56384 ν 1 = 60.7 r 2 = 319.640* d 2 = 9.00 n 2 =1.51633 ν 2 = 64.1 r 3 = −478.031 d 3 = 16.76 r 4 = 96.413 d 4 = 8.60 n3 = 1.51823 ν 3 = 58.9 r 5 = 472.518 d 5 = 3.11 r 6 = −495.228 d 6 =3.60 n 4 = 1.74950 ν 4 = 35.3 r 7 = 135.791 d 7 = 4.08 r 8 = 71.132 d 8= 8.40 n 5 = 1.48749 ν 5 = 70.2 r 9 = 245.218 d 9 = 0.80 r10 = 51.628d10 = 5.30 n 6 = 1.67270 ν 6 = 32.1 r11 = 40.134 d11 = 39.15 r12 =1141.040 d12 = 1.80 n 7 = 1.43387 ν 7 = 95.1 r13 = 56.180 d13 = 22.84r14 = ∞ (Stop) d14 = 10.50 r15 = 90.269 d15 = 1.30 n 8 = 1.80518 ν 8 =25.4 r16 = 34.628 d16 = 4.70 n 9 = 1.48749 ν 9 = 70.2 r17 = −87.040 d17= 0.50 r18 = 68.906 d18 = 3.85 n10 = 1.76182 ν10 = 26.5 r19 = −59.203d19 = 1.30 n11 = 1.80400 ν11 = 46.6 r20 = 32.357 d20 = 3.41 r21 =−74.136 d21 = 1.30 n12 = 1.80400 ν12 = 46.6 r22 = 131.844 d22 = 1.53 r23= 75.659 d23 = 6.20 n13 = 1.63980 ν13 = 34.5 r24 = −31.349 d24 = 1.40n14 = 1.80400 ν14 = 46.6 r25 = −170.115 d25 = 14.65 r26 = 82.731 d26 =6.60 n15 = 1.51633 ν15 = 64.1 r27 = −106.118 d27 = 0.72 r28 = ∞ d28 =2.20 n16 = 1.51633 ν16 = 64.1 r29 = ∞ *Diffractive Surface C₁ =−4.22716D−05 C₂ = 4.71244D−10

Numerical Example 2: f = 51.50  Fno = 1:1.46  2ω = 45.57° r 1 = 50.061 d1 = 4.15 n 1 = 1.78000 ν 1 = 50.0 r 2 = 243.413 d 2 = 0.10 r 3 = 30.828d 3 = 3.68 n 2 = 1.88500 ν 2 = 41.0 r 4 = 45.316 d 4 = 1.98 r 5 = 57.891d 5 = 3.40 n 3 = 1.65070 ν 3 = 31.8 r 6 = 20.182 d 6 = 5.87 r 7 = ∞(Stop) d 7 = 10.63 r 8 = −19.290 d 8 = 3.40 n 4 = 1.79528 ν 4 = 28.1 r 9= 131.601 d 9 = 6.77 n 5 = 1.79558 ν 5 = 48.3 r10 = −32.350* d10 = 1.10r11 = −106.926 d11 = 5.26 n 6 = 1.88430 ν 6 = 40.4 r12 = −36.060 d12 =0.10 r13 = 82.121 d13 = 3.60 n 7 = 1.88500 ν 7 = 41.0 r14 = 455.723*Diffractive Surface C₁ = −2.252500D−04 C₂ = 4.0128D−07 C₃ =−4.53740D−10

Numerical Example 3: f = 24.61  Fno = 1:1.45  2ω = 82.64° r 1 = 62.320 d1 = 2.80 n 1 = 1.69680 ν 1 = 55.5 r 2 = 31.224 d 2 = 5.77 r 3 = 58.654 d3 = 2.30 n 2 = 1.69680 ν 2 = 55.5 r 4 = 32.021 d 4 = 6.94 r 5 = 220.248d 5 = 4.36 n 3 = 1.71300 ν 3 = 53.8 r 6 = −101.757 d 6 = 4.09 r 7 =72.682 d 7 = 2.96 n 4 = 1.84666 ν 4 = 23.8 r 8 = 340.356 d 8 = 1.70 n 5= 1.49700 ν 5 = 81.6 r 9 = 23.248 d 9 = 12.21 r10 = 30.622 d10 = 6.82 n6 = 1.80400 ν 6 = 46.6 r11 = 58.996 d11 = 0.15 r12 = −1174.273 d12 =1.48 n 7 = 1.72825 ν 7 = 28.5 r13 = 38.385 d13 = 4.63 r14 = ∞ (Stop) d14= 7.99 r15 = 16.394 d15 = 4.66 n 8 = 1.84666 ν 8 = 23.9 r16 = −37.366**d16 = 0.15 r17 = −201.177 d17 = 7.04 n 9 = 1.60311 ν 9 = 60.7 r18 =−23.231* d18 = 0.15 r19 = −86.014 d19 = 5.55 n10 = 1.77250 ν10 = 49.6r20 = −29.191 *Diffractive Surface **Aspheric Surface

A =  0.00000D+00 B =  2.10265D−05 C =  1.79508D−08 D = −1.59961D−11 E =−1.82408D−13 F =  2.10282D−16 C₁ = −4.42491D−04 C₂ =  3.53506D−07 C₃ = 3.46931D−11 C₄ =  2.27434D−12 C₅ = −2.17625D−15 C₆ =  9.09469D−18 C₇ =−2.48396D−20 C₈ =  9.80318D−23 C₉ = −1.98660D−24 C₁₀ =  8.60378D−27

TABLE 1 Condition Numerical Example No. Factor 1 2 3 (1) |D/R| 0.0261.172 2.801 (2) C₁ · P 6.29847D−09 −6.20948D−06 −1.14875D−05

With the above-described elements defined as set forth in each of theembodiments, it is possible to attain an optical system having such highoptical performance that, when effecting achromatism by combining adiffractive optical element and a refractive optical element, thediffraction efficiency excellent over the entire image plane can beobtained even if light fluxes which are to reach respective positions ofthe image plane overlap each other greatly on a diffractive opticalsurface.

What is claimed is:
 1. An optical system, comprising: a diffractiveoptical element having a diffraction grating provided, on a lens surfacehaving curvature, in a concentric-circles shape rotationally-symmetricalwith respect to an optical axis, wherein a sign of the curvature of thelens surface having said diffraction grating provided thereon is thesame as a sign of a focal length, in a design wavelength, of a systemcomposed of, in said optical system, a surface disposed nearest to afront side to a surface disposed immediately before the lens surfacehaving said diffraction grating thereon, and is different from a sign ofa distance from the optical axis to a position where a center ray of anoff-axial light flux enters the lens surface having said diffractiongrating provided thereon, wherein the following condition is satisfied:|DL/R|<0.3 where DL is a distance from (a) the apex of an imaginary coneformed by extending a non-effective surface of said diffraction gratingto (b) the center of curvature of the lens surface having saiddiffraction grating provided thereon, and R is a radius of curvature ofthe lens surface having said diffraction grating provided thereon.
 2. Anoptical system according to claim 1, wherein said optical systemsatisfies the following condition: |D/R|<5 where D is a distance fromthe center of curvature of the lens surface having said diffractiongrating provided thereon to a focus, at the design wavelength, of thesystem composed of, in said optical system, the surface disposed nearestto the front side to the surface disposed immediately before the lenssurface having said diffraction grating provided thereon.
 3. An opticalsystem according to claim 1, wherein said optical system satisfies thefollowing condition: C ₁·P<0 where P is a refractive power of the lenssurface having said diffraction grating provided thereon, and C₁ is aphase coefficient for a second-degree term when a phase shape of saiddiffraction grating is expressed by the following equation:φ(Y)=(2π/λ₀)(C ₁ Y ² +C ₂ Y ⁴ +C ₃ Y ⁶+. . . ) where Y is the height ina vertical direction from the optical axis, λ₀ is the design wavelength,and C_(i) is a phase coefficient (i=1, 2, 3 . . . ).
 4. An opticalsystem according to claim 1, wherein said diffraction grating is alaminated diffraction grating.
 5. An optical system according to claim4, wherein said laminated diffraction grating is an adjacently-laminateddiffraction grating in which two diffraction gratings are disposedadjacent to each other across an air layer.
 6. An optical apparatuscomprising an optical system according to claim 5, wherein saidadjacently-laminated diffraction grating is provided between twoadjacent lens surfaces having substantially the same curvature, and iscomposed of three layers, including, in order from the front side, afirst layer, a second layer and a third layer, said second layer beingthe air layer, wherein said optical apparatus is selected from the groupof optical apparatuses consisting of an image pickup apparatus, a filmcamera, a video camera, a digital camera, an observation apparatus, atelescope, a binocular, a projection exposure apparatus, and a stepperfor manufacturing a semiconductor device.
 7. An optical system accordingto claim 5, wherein each of said two diffraction gratings of saidadjacently-laminated diffraction grating are formed withultraviolet-curable plastic.
 8. An optical system according to claim 1,wherein said diffraction grating is a blazed-type diffraction grating.9. An optical apparatus comprising: an optical system comprising adiffractive optical element having a diffraction grating provided, on alens surface having curvature, in a concentric-circles shaperotationally-symmetrical with respect to an optical axis, wherein a signof the curvature of the lens surface having said diffraction gratingprovided thereon is the same as a sign of a focal length, in a designwavelength, of a system composed of, in said optical system, a surfacedisposed nearest to a front side to a surface disposed immediatelybefore the lens surface having said diffraction grating thereon, and isdifferent from a sign of a distance from the optical axis to a positionwhere a center ray of an off-axial light flux enters the lens surfacehaving said diffraction grating provided thereon, wherein the followingcondition is satisfied: |DL/R|<0.3 where DL is a distance from (a) theapex of an imaginary cone formed by extending a non-effective surface ofsaid diffraction grating to (b) the center of curvature of the lenssurface having said diffraction grating provided thereon, and R is aradius of curvature of the lens surface having said diffraction gratingprovided thereon.
 10. An optical apparatus according to claim 9, whereinsaid optical apparatus is selected from the group of optical apparatusesconsisting of an image pickup apparatus, a film camera, a video camera,a digital camera, an observation apparatus, a telescope, a binocular, aprojection exposure apparatus, and a stepper for manufacturing asemiconductor device.
 11. An optical system, comprising: a diffractiveoptical element having a diffraction grating provided, on a lens surfacehaving curvature, in a concentric-circles shape rotationally-symmetricalwith respect to an optical axis, wherein a sign of the curvature of thelens surface having said diffraction grating provided thereon is thesame as a sign of a focal length, in a design wavelength, of a systemcomposed of, in said optical system, a surface disposed nearest to afront side to a surface disposed immediately before the lens surfacehaving said diffraction grating thereon, and is different from a sign ofa distance from the optical axis to a position where a center ray of anoff-axial light flux enters the lens surface having said diffractiongrating provided thereon, wherein an apex of an imaginary cone formed byextending a non-effective surface of said diffraction grating is locatedadjacent to the center of curvature of the lens surface having saiddiffraction grating provided thereon, wherein said diffraction gratingis a laminated diffraction grating, and wherein said laminateddiffraction grating is an adjacently-laminated diffraction grating inwhich two diffraction gratings are disposed adjacent to each otheracross an air layer.
 12. An optical system according to claim 11,wherein each of said two diffraction gratings of saidadjacently-laminated diffraction grating are formed withultraviolet-curable plastic.
 13. An optical apparatus, comprising: anoptical system according to claim
 11. 14. An optical apparatus accordingto claim 13, wherein said optical apparatus is selected from the groupof optical apparatuses consisting of an image pickup apparatus, a filmcamera, a video camera, a digital camera, an observation apparatus, atelescope, a binocular, a projection exposure apparatus, and a stepperfor manufacturing a semiconductor device.
 15. An optical apparatusaccording to claim 14, wherein said adjacently-laminated diffractiongrating is provided between two adjacent lens surfaces havingsubstantially the same curvature, and is composed of three layers,including, in order from the front side, a first layer, a second layerand a third layer, said second layer being the air layer.
 16. An opticalsystem, comprising: a diffractive optical element having a diffractiongrating provided, on a lens surface having curvature, in aconcentric-circles shape rotationally-symmetrical with respect to anoptical axis, wherein a sign of the curvature of the lens surface havingsaid diffraction grating provided thereon is the same as a sign of afocal length, in a design wavelength, of a system composed of, in saidoptical system, a surface disposed nearest to a front side to a surfacedisposed immediately before the lens surface having said diffractiongrating thereon, and is different from a sign of a distance from theoptical axis to a position where a center ray of an off-axial light fluxenters the lens surface having said diffraction grating providedthereon, wherein the following condition is satisfied: |DL/R|<0.3 whereDL is a distance from (a) the apex of an imaginary cone formed byextending a non-effective surface of said diffraction grating to (b) thecenter of curvature of the lens surface having said diffraction gratingprovided thereon, and R is a radius of curvature of the lens surfacehaving said diffraction grating provided thereon, wherein the apex ofthe imaginary cone is located on the optical axis, and wherein saiddiffraction grating is entirely a blazed-type diffraction grating.